What does an iron meteorite look like inside?
Inside, an iron meteorite is solid, silvery metal. When cut, polished and etched, many show a criss-cross Widmanstätten pattern or fine Neumann lines.
From the outside, an iron meteorite usually looks like a rusty, lumpy or thumb-printed piece of metal. Inside, it is quite different.
Cut and polished. A fresh cut surface of an iron meteorite is bright, silvery metal, similar to polished steel. It may contain inclusions: bronze-coloured nodules of troilite, an iron sulfide; black grains of graphite; or silvery-yellow crystals of schreibersite, an iron-nickel phosphide.
Etched. When a polished surface is treated with a weak acid solution, usually by experts, a pattern appears because different iron-nickel minerals react differently.
- Most irons, called octahedrites, show the Widmanstätten pattern: crossing bands of kamacite, a low-nickel iron alloy, bordered by thin taenite, a high-nickel alloy, with fine-grained plessite filling some spaces. The bands may be broad or very fine, depending on nickel content and cooling rate. - Hexahedrites, low in nickel, are made almost entirely of kamacite and show fine parallel Neumann lines, formed by shock. - Ataxites, high in nickel, show no visible pattern.
Why it matters. The Widmanstätten pattern forms only by extremely slow cooling over millions of years, deep inside an asteroid core. It cannot be produced in manufactured iron, so it is one of the clearest signs of a genuine iron meteorite. Fakes that imitate it exist, so expert examination is still important.
Safety. Cutting and etching iron meteorites requires proper equipment, ventilation and protective gear. Home experiments with acids are not advised, especially for children.